Encapsulant materials are used in a variety of applications to isolate components, areas, or other materials from potentially stressful conditions that can adversely affect the performance of a device. For example, the performance of photovoltaic (PV) modules may decrease over time as water penetrates the module and corrodes the metallic components essential for module function. In the absence of water, corrosion occurs relatively slowly because by-products are less able to diffuse away from a surface to allow the corrosion process to progress. Furthermore, water is known to help catalyze some oxidative reactions.
The PV industry has long-recognized the dramatic effect that corrosion has on module performance. Today, PV modules typically include a polymeric encapsulant material to isolate the silicon components from the ever-present potentially adverse conditions created by various sources of water, including rain, snow, and condensation. The isolation created by the encapsulant protects the PV components from the potential for corrosion and provides additional benefits, including mechanical support, electrical insulation and protection from mechanical damage.
Polymeric encapsulants provide the desired isolation by bonding to a surface and limiting access to the protected areas and/or components. For example, encapsulants used in PV modules are typically bonded to one or more glass sheets to isolate the solar cells, or cell strings, from water in the module's environment. The ability of a polymeric material to protect a surface is thus highly dependent on its ability to bond to a surface and limit access to corrosion sites. Therefore, a strong correlation exists between corrosion protection and adhesive strength.
The dominant encapsulant used in the PV industry is based on a random copolymer consisting of about 67 wt % polyethylene and 33 wt % poly vinyl acetate. Polyethylene was chosen because it is a very simple and inexpensive polymer. When used alone, however, it is typically an opaque or translucent (depending on the polymerization conditions) semicrystalline polymer with a modulus too high to mechanically protect a PV device. Poly vinyl acetate is a transparent, amorphous polymer, but it has a glass transition temperature (Tg) of about 35° C., making it too brittle and/or noncompliant under typical environmental exposure. Therefore, a small amount of vinyl acetate is added to polyethylene to break up the crystallites, producing a semicrystalline, highly transparent material. Typically, 33 wt % vinyl acetate is copolymerized with ethylene to get a good mix of properties such as a high optical transmission and a low Tg.
Over the last several decades, EVA has emerged as the dominant encapsulant material used in PV devices. The adoption of EVA as a de facto standard occurred not because it had the best combination of properties, but because it was inexpensive and readily available. Early modules constructed with EVA demonstrated severe failure within a few years of putting the modules in use because of yellowing of the encapsulant. Improvements to EVA have been developed, including formulations with antioxidant and ultraviolet (UV) absorbers, that provide encapsulant materials that will not significantly yellow over the 20- to 30-year lifetime of a module.
Despite these improvements, EVA still has several drawbacks that affect its performance as an encapsulant material, particularly in PV modules. For example, EVA suffers from non-ideal mechanical and thermal properties, a high diffusivity for water, and acetic acid by-product production. Furthermore, the newer thin-film technologies that are rapidly being developed in the PV industry may be more sensitive to the shortcomings of EVA. As crystalline silicon wafers become thinner, the mechanical properties of EVA may also prove insufficient.
Furthermore, EVA was designed to be used on the front side of cells where high light transmission is required. It is also routinely used on the back side of cells where light transmission is not necessary. In these applications, a white sheet of Tedlar (or another reflective material) is commonly laminated to the back to improve performance by reflecting back the light that initially shines between the cells. This PV module construction method is common because sufficient research into inexpensive non-transparent alternatives has not produced adequate materials that the industry trusts. When the requirement for optical transmission is removed, the use of a much wider variety of alternative encapsulant materials becomes feasible.
The PV industry, generally speaking, is under significant pressure to reduce the cost of manufacturing PV modules. Before PV modules—and the renewable energy they deliver—can enjoy widespread adoption, the manufacturing process must be refined to a point at which the product, PV modules, has desirable price points as compared to conventional energy sources. In this current environment, there is great interest in new technologies that realize efficiencies in the module manufacturing process. Encapsulants, as a component in PV modules, provide an opportunity to realize efficiencies in the module manufacturing process and overall module cost. An encapsulant providing even a minimal cost savings is expected to be well-received in the current environment, particularly if the encapsulant also provides beneficial technical properties.
As the ability to isolate components and areas from potentially adverse conditions is not absolute, there is a continuing need for improved encapsulant materials. The PV industry currently has a particularly well-defined need for such improved materials.